The jail sentence imposed for the one-hit killing of teenager Thomas Kelly is not long enough to act as a deterrent, a Sydney court has heard.

The New South Wales Court Of Criminal Appeal is hearing a Director of Public Prosecutions' (DPP) appeal against the minimum four-year sentence given to Kieran Loveridge in November 2013.

Loveridge randomly punched Mr Kelly in the head in July last year as he walked with his girlfriend and spoke on his mobile phone in Kings Cross.

The single punch knocked Mr Kelly to the ground and he died from head injuries in St Vincent's Hospital two days later.

Loveridge was originally charged with murder, but prosecutors accepted his guilty plea to the lesser charge of manslaughter.

The lawyer for the DPP Nicole Noman SC has told the court the sentence handed down by Justice Stephen Campbell is not long enough to act as a deterrent and there is a "requirement to maintain public confidence".

She said Loveridge displayed a sustained level of criminality the night he hit Mr Kelly and also assaulted a number of other people.

Ms Noman said Loveridge had told colleagues he wanted to "bash someone".

In asking for the appeal to be dismissed, Loveridge's lawyer, Phillip Boulten SC said his client has "been able to articulate remorse" and at the time of the offence "was an 18-year-old who was very drunk".

Mr Boulten said the sentencing exercise was a difficult one for Justice Campbell who handled it carefully.

"The total affective sentence was in the range appropriately available," he said.

He noted that up until the night he hit Mr Kelly, Loveridge was behaving himself and had turned his life around.

Mr Boulten said Loveridge also has insight into his substance abuse behaviour.

Members of the Kelly family were visibly upset during the hearing.

Outside court Mr Kelly’s father Ralph said the family looks forward to seeing reform to laws governing drunken violence while his mother Kathy said her son should still be alive.

"Alcohol is not an excuse...there are too many victims of senseless violence and the community needs to continue ask for change," she said.